Embodiments of the inventive subject matter relate to image signal processing circuits and electronic devices incorporating the same and, more particularly, to operational amplifier circuits used to process image signals from, for example, a pixel sensor array.
An image sensor is a device that captures an image using a semiconductor reaction to light. Recently, with the development of complementary metal oxide semiconductor (CMOS) technology, CMOS image sensors are being widely used. Some conventional CMOS image sensors use correlated double sampling (CDS) and output a signal resulting from the CDS, e.g., a difference between a reset signal and an image signal, in a digital format.
A CMOS image sensor circuit may include a programmable gain amplifier (PGA), an analog-to-digital converter (ADC circuit), and a sense amplifier (SA). The PGA, the ADC circuit, and the SA typically include operational amplifier circuits. The ADC circuit may include a sample-and-hold amplifier (SHA) and a digital-to-analog converter (DAC). The SHA and the DAC may also include operational amplifier circuits.
Such operational amplifier circuits may include a negative feedback amplifier. The gain of a negative feedback amplifier generally depends on circuit components forming the operational amplifier circuit. When the gain of the negative feedback amplifier approaches infinity, the output of the operational amplifier circuit may oscillate.
The stability of the operational amplifier circuit is generally influenced by phase margin (PM). FIGS. 1A and 1B are Bode plots of frequency response of a conventional operational amplifier circuit. Referring to FIGS. 1A and 1B, frequencies at which feedback loop gain and phase are 1 and −180 degrees, respectively, are important to the stability of the operational amplifier circuit. The frequency at which the feedback loop gain is 1 is referred to as a gain crossover. The frequency at which the feedback loop phase is −180 degrees is referred to as a phase crossover.
It is desirable that the gain crossover be less than the phase crossover in order to provide stable operation of the operational amplifier circuit. Stability can be improved by shifting the phase crossover away from the origin or shifting the gain crossover toward the origin. The stability of a system can be measured in terms of PM. The PM is defined as PM=180°+∠βH(ω=ω1), where ω1 is the gain crossover frequency.
When the gain crossover is shifted toward the origin, the stability of the operational amplifier circuit is increased, but the bandwidth is generally decreased. As a result, high-frequency signals are attenuated. When the bandwidth is increased, the operational amplifier circuit can realize higher speed. When an operational amplifier circuit that can realize a wide bandwidth is used, high sampling speed may be achieved in communications and image processing applications, such as in mobile communication equipment, asynchronous digital subscriber loop (ADSL), digital camcorders, and high-definition television (HDTV) and imaging systems.